`MarkovChain` validators and `recursive_tree_part`

[cdonnay]

The edit to chain.py ensures that if a user only inputs one constraint to a MarkovChain, it will still be able to print the correct error message should the chain fail. The error message relies on the constraints being turned into a Validator object, and previously if the user input a single constraint it did not get turned into a Validator.

The edit to tree.py fixes an issue that occasionally caused recursive_tree_part to generate plans that were imbalanced in population. Here is an example to show why the previous computation was incorrect. Suppose the ideal size is 100, epsilon = .02, and the previous district had size 99. Then the debt=-1, the min_pop=99, and max_pop=102. The pop_target = 100.5, and the new value for epsilon is .015. But then the lower end of the population range is actually (100.5)*(1-.015)=98.9925, and the upper end is 100.5**1+.015)=102.0075. The only change required to fix this is to compute the new value for epsilon using the new population target, not the ideal size.

[codecov[bot]]

Codecov Report

Merging #416 (6751a83) into main (1b8430d) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

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